1. Field of the Invention
The present invention relates to a developing-equipment switching apparatus utilized in an electrophotographic color image forming apparatus having a plurality of developing implements such as a color copying machine, a color printer and the like and more particularly to a developing-equipment switching apparatus for switching developing implements from one to another in an image forming apparatus for forming color images by switching the developing implements facing a latent image carrier such as a photosensitive drum from one to another so as to form a toner image of each color.
2. Description of the Related Art
Electrophotographic color image forming apparatus have increasingly been made compact now. For this reason, the tendency is for, in place of the conventional fixed type developing apparatus, a so-called moving developing apparatus to be adopted, the developing apparatus being adapted to switching developing implements facing such a latent image carrier by making a moving member set adjacent to the latent image carrier hold a plurality of developing implements and moving the moving member. Moreover, it has also been proposed to employ a rotary retainer capable of rotation so as to switch developing implements and a linear moving member capable of linear movement so as to switch developing implements.
In the image forming apparatus for switching developing implements like that in comparison with the image forming apparatus provided with a plurality of developing implements around a latent image carrier, the number of members arranged around the latent image carrier is reducible, so that not only the whole apparatus but also the latent image carrier itself can be downsized. Since all the developing implements face the latent image carrier at the same position, moreover, the advantage is that a signal for controlling the operation of the developing implement, for example, a signal for controlling developing bias application timing and such a member as a magnetic roll can be set for common use.
As Japanese Patent Examined Publication No. Hei 2-13304 discloses, a conventional developing-implement switching apparatus of the sort mentined above comprises a rotatably-supported rotary retainer for holding four developing implements in such a way as to make each of the developing implements face a latent image carrier when the rotary retainer rotates, a driving motor for generating driving force, control means for controlling the rotation of the driving motor, a primary gear anchored to the rotary shaft of the driving motor and rotated together with the rotary shaft, and a secondary gear fitted to the rotary retainer and rotated together with the rotary retainer, wherein the rotary retainer is rotated by transmitting the torque of the driving motor to the rotary retainer via the primary and secondary gears, whereby the developing implements facing the latent image carrier are switched from one to another.
In order to equalize the developing conditions of all the developing implements in the aforesaid developing-equipment switching apparatus, each of the developing implements needs positioning precisely. Therefore, the target value of the rotational rate of the rotary retainer in accordance with the space between developing implements to be rotated and simultaneously the rotary shaft of the driving motor is fitted with an encoder whose rotation is read by a detection means. Further, the rotational rate of the rotary retainer is calculated on the basis of intervals at which the codes of the encoder are generated and the driving motor is controlled so that the calculated rate conforms to the aforementioned target value.
The encoder is supplied with a plurality of marks disposed in the rotational direction thereof and the detection means generates a code each time the mark arrives at a proper facing position.
Notwithstanding, the target value of the rotational rate of the rotary retainer has to be classified into those at the time of acceleration, constant rate operation and reduction to actualize the control operation above, which has the control operation complicated. In addition, the developing-equipment switching apparatus tends to become complicated in construction and thus costly because it needs a large capacity memory for storing the rates, an encoder, a detection means, a timer, a rate counter, a control CPU and the like.